1. Field of the Invention
This invention relates generally to ultrasonic probes for transmitting and receiving ultrasonic waves for ultrasonic diagnosic apparatus, and, more particularly, to ultrasonic probes having matrix arrays of transducers.
2. Description of the Prior Art
Ultrasonic diagnostic apparatus are used to obtain tomogram images through the detection of reflected waves generated by the scanning of internal organs of a subject's body. These apparatus have come rapidly into wide use due to their real time capabilities and the superior diagnostic results obtained. Mechanical scanning and electronic scanning are among the types of scanning methods. Mechanical scanning is effected by mechanical movement of ultrasonic transducers. Electronic scanning is effected by electronic switching of a matrix array of transducers and control of the delay time. Electronic scanning has become the most popular due to its real-time operation and increased resolution. Electronic scanning systems may be classified into linear scanning and sector scanning types.
Techniques to increase the resolution in the scanning direction include a receiving dynamic focusing method. The dynamic focusing method switches focal points of the ultrasonic beams according to times corresponding to the depth into the subject's body at the time the beam is received, and combines pictures near focus points with repeated transmitting and receiving for different focus points. Wide range is achieved using multi layers as the matching layer of ultrasonic transducers in the depth direction of the subject's body. As a result, an increase of resolution can be achieved.
However, an acoustic lens has been used to focus ultrasonic beams to one point in the vertical direction to the scanning plane, e.g., the slicing direction. The width of the ultrasonic beams spreads on remote sides of the focus points. Good images are obtained near the focus points of the acoustic lens and are integrated by the width of ultrasonic beams in the slicing direction. However, the image fades on remote sides of the focus points, where the width of the ultrasonic beams spreads. As a result, microscopic structures of fine blood vessels, and the like, are not shown distinctly.
Attempts have been made to increase the resolution in the slicing direction using matrix array transducers to overcome these problems. However, a matrix array of transducers required too many transducers.
In addition, unnecessary vibration modes appear in directions other than the depth direction with general matrix array transducers, and it is difficult to remove these unnecessary vibrations, if the cutting width of the transducers approaches the thickness thereof. Conventional linear type transducers are cut fine enough, and are externally electrically. If these same methods are applied to matrix array transducers, the transducers also have to be cut very fine in the slice direction. In that case, the number of transducers becomes enormous.
As a result, manufacture has been difficult and has taken a long time. Also, large loads must be connected between the transducers and the electric circuits using leads, and transducers are expensive. In the case of matrix array transducers, the above-mentioned problems may be reduced, if individual electrodes are divided into the transducers without cutting the piezoelectric substrate. However, crosstalk between transducers through the uncut piezoelectric substrate is generated and the signal to noise ratio decreases.
It is very difficult to suppress unnecessary vibration modes by the finer division of transducers in the conventional ultrasonic probes having matrix array transducers, because the number of transducers is so great. If the transducers are divided only by individual electrodes, crosstalk characteristics will be poor.